Titanium dioxide (TiO2), also referred to as titania, is found in three known crystal forms, rutile, anatase and brookite. Anatase and rutile can be used industrially. There are various known methods of synthesis, compositional variants including admixtures, and thermal processing which can modify define the crystalline form(s) obtained.
Titania is the most widely used white pigment for the dye and paint industry, ceramics, paper, rubber, and plastic manufacturing. Titania is also used in the ointment and other cosmetics production, especially for UV protection.
Photocatalytic activity is generally the most important feature of Titania. Photocatalytic reactions do not lead to photocorrosion of the reagents and their composition remains unchanged unlike photochemical reactions, where semiconducting reagents undergo photocorrosion. Photochemical reagents absorb light and promote reactions between various substances in gaseous or liquid phases, or induce electrical current. Many semiconductors (including titania) exhibit such kind of photochemical activity. Semiconducting photocatalysis is a complex phenomenon with numerous promising spectro-optical, thermodynamic, kinetic, electrophysical and some other fundamental prospects. For instance, titania can be used as a basic material for the highly effective photocatalytic systems for transformation, conservation and utilization of the solar energy and for the hazardous waste neutralization and for other environment preservation solutions. Titania products also bring good prospects for the low-tonnage chemistry, for design and production of multi-functional materials (for example, materials containing thin-precipitated layer of nano-particles on various substrates), for production of the optical sensors and materials with non-linear optical properties.
These reasons promote numerous photocatalytic investigations. There has been reported water decomposition, which occurs on the surface of titania and produces ecologically friendly molecular hydrogen which can be used as a fuel. There are many successful investigation projects in the field of photocatalysis, as well as many surveys and general works. However, very low quantum yield of most photocatalytic systems is a serious shortcoming, which limits potential applications.
Simple photocatalytic semiconducting system includes donor (D) and acceptor (A) parts acting on the photocatalyst (e.g. titania). A closed photocatalytic loop can generally work only if acceptor accepts the excited electron from the conduction band (e−+A→A−) and the hole would transfer to donor (h++D→D+) after photogeneration of the electron-hole couple
Further transformation of the intermediates A− and D+ can run even without light and photocatalyst. Energetic properties of the photocatalytic system should correspond to each other. The electron-hole reactions can run if they are thermodynamically allowed, e.g. potential of the conductivity band should be more negative than the oxidation potential of D (ECB<EAred and EVB>EDox). Efficiency of reactions e−+A→A− and h++D→D+ (and the photocatalytic process itself) should rise as the gaps ΔEred=EAred−ECB and ΔEox=EVB−EDox widen.
The photogenerated electrons and holes can recombine. The recombination process competes with the above mentioned redox scheme and significantly lowers efficiency of any semiconducting photocatalyst (including titania). Therefore, photocatalytic systems are preferably modified (e.g. by insertion of the electrons and holes carriers, deposition of metals or metal oxides on semiconductors, using double-semiconductor heterostructures) to reduce recombination.
Titania can be obtained either in the crystalline or hydrated form through various methods and from various source compounds. Hydrolysis of the aqueous solutions of Ti(IV), hydrolysis of vapor or aerosol, thermodecomposition of alcoholates or coordination compounds of Ti(IV), and high temperature hydrolysis of TiCl4 are the most widely used methods for titania production. There are many industrial methods for titania production.
Titania can be used as a light-sensitive component for photo-layers and dielectric materials or as a photocatalyst for some redox reactions. Such compounds generally provide high photocatalytic activity and dispersibility.
Most industrial samples of titania comprise coarsely dispersed particles with low photocatalytic activity. There are some methods of production of fine disperse titania with reasonably high photosensitivity. However, such methods are generally inconvenient and laborious.
There is a method of production of superfine titanium dioxide disclosed in U.K. Patent 1052896 which discloses burning of titanium tetrachloride (preliminary heated up to 350° C.) in the gas mixture, which contains oxygen (or carbon dioxide) and hydrogen at 1200-1400° C. Oxygen content in the gas mixture is disclosed to be slightly stoichiometrically excessive relative to the hydrogen content in the gas mixture. In this way, the rutile type of fine titanium dioxide is obtained. However, the photosensitivity of the resulting product is very low and its specific photocatalytic activity (determined through the reaction of the methylene blue reduction) is only about 2.5-3.0×10−5 mg/ml·min·m2 at room temperature.